Just discovered this thread after following along your rust repair one Coultl, which I enjoyed a lot.
Very cool work on the 3D scanning, design and printing! And it will be immensely satisfying to eventually drive a car that you rebuilt from front to back yourself.
One thing I would caution on is using FDM for structural parts that need isotropic strength, so as a pressurised intake manifold. Even with a 3mm wall thickness and a strong material like PPA-CF, the layer adhesion is most likely going to be an issue. The added CF works well for helping appearance, stiffness, and preventing warping, but it also weakens layer adhesion since it also acts kind of as a contaminant where the plastics wants to stick to itself. The newer core filaments such as from Siraya Tech partially address this issue, but strength in Z is still much lower than along the layer.
I wonder how much SLS printing this big piece in China would cost, either in nylon or ideally some form of metal? Alternatively, you can also try to find somebody with a printer with a heated chamber and an annealing oven, both of those can also help the parts become more isotropic.
I would definitely build a testing rig you can pressurise to play around with it, before it goes on a very expensive Porsche motor. Please keep us posted on how this testing goes, hopefully I am completely wrong and it all works out sweetly in the end.
Cheers,
Lukas